Ion implantation is a standard technique for introducing conductivity—altering impurities into semiconductor wafers. A desired impurity material may be ionized in an ion source, the ions may be accelerated to form an ion beam of prescribed energy, and the ion beam may be directed at a front surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
An ion implanter may include a scanner for deflecting or scanning the ion beam in at least one direction at a scan frequency in order to distribute the ion beam over the front surface of the wafer. The scanner may be an electrostatic scanner or a magnetic scanner as are known in the art. The ion beam may be distributed over the wafer area by the beam scanning alone or by a combination of beam scanning and wafer movement. In one ion implanter, the scanner may scan the beam in one direction and a drive system may translate the wafer in a direction orthogonal to the scan direction to distribute the ion beam over the front surface of the wafer.
Introducing the ions at a specified depth and density into the wafers, which may be a uniform depth and density, is important to ensure that the semiconductor device being formed operates within specification. One factor that can affect the uniformity of the dose into the wafer is the ion beam current. An unexpected fluctuation in ion beam current, e.g., a glitch, may degrade the resulting uniformity of the impurity dose. Accordingly, various ion beam current monitoring systems have been developed.
One conventional ion beam current monitoring system includes one or more Faraday sensors disposed about the perimeter of the wafer to monitor ion beam current as the scanned ion beam is scanned off the front surface of the wafer. When such a beam current monitoring system is utilized with a scanned ion beam, a relatively higher scan frequency would provide more frequent beam current measurements than a comparatively lower scan frequency. A relatively higher scan frequency would also provide more passes of the ion beam over the front surface of the wafer assuming a scan of the beam in one direction and a constant velocity of wafer movement in a direction orthogonal to the scan direction. More passes of the ion beam over the front surface of the wafer may assist with achieving dose uniformity goals for certain ion beams.
Therefore, a scanner in one conventional ion implanter uses a relatively higher scan frequency on the order of about 1 kilohertz (kHz). This higher scan frequency may be adjusted within a modest range about 1 kHz for dose uniformity purposes. This relatively higher scan frequency provides the aforementioned benefits for high energy ion beams. However, the higher scan frequency for low energy ion beams may not provide sufficient time for beam neutralization efforts. This may then lead to decreased beam current for the low energy ion beams, a decreased success rate for uniformity set up for low energy ion beams, and an increase in uniformity tune time during beam setup.
Accordingly, there is a need for an ion implanter with a variable scan frequency that enables a higher scan frequency to be utilized for high energy ion beams and a lower scan frequency to be utilized for low energy ion beams.